1. Field of the Invention
This invention pertains generally to noise cancellation methods and devices, and more particularly to a method and apparatus for actively suppressing noise vibrations transmitted from a medical instrument through bone structure and intracranial tissue in the head.
2. Description of the Background Art
In 1844, when Horace Wells introduced anesthesia, including nitrous oxide, and later in 1896 when sulfuric ether was introduced, the discomfort experienced by patients receiving dental health care sharply declined. Today, general anesthesia is considered dentistry's greatest discovery. However, in 1872 when S.S. White Company introduced the first electric drill which was invented by George F. Green, the discomfort caused by drill noise began to increase and continued to do so with further increases in drill speed. The level of discomfort reached a maximum for many patients when the high-speed pneumatic turbine drills were introduced in the 1960's. Today, dentists report that the chief complaint they receive from their patients is--aside from the hypodermic needle--the discomfort from noise caused by either the high-frequency whine of the high-speed (300,000-400,000 maximum RPM) drills or, for some, the chatter and vibration of the so-called low-speed (around 30,000 maximum RPM) drills.
Some reduction of perceived drill noise can be accomplished by muffling the patient's ears, or by using "active headsets" which both muffle the patient's ears and perform noise cancellation of the sound which propagates through the air to the patient's ears. However, even with total suppression of the pressure waves arriving at the ear due to drill noise propagating through the air, dental patients experience only negligible reduction in discomfort because a great deal of the drill noise perceived arrives at the inner ear from propagation paths through the head. That is, the vibration induced in the tooth by the drill enters the bone and propagates along the skull and through the skull interior to the temporal bone and finally to the inner, middle, and outer ears, each of which contributes to the vibrations in the cochlea which are converted to nerve impulses that are sent on to the brain.
Vibrational energy can travel along two different pathways to the cochlea: through the bone, and through the skull interior. Energy transmitted by these pathways is brought together at the temporal bone. From that point, there is a signal line that goes to three separate points of input: (1) the walls of the external auditory canal, (2) the middle ear ossicle, and (3) the cochlear capsule and its contained fluid. In addition, the cochlea has an independent input from the skull interior via what is referred to as the "third window". At each of these points, which represent separate inputs to the outer, middle, and inner ears, respectively, the responses are altered by a number of modifying factors. Specifically, the walls of the external canal radiate sound into its lumen (air), the modifying fact being the external opening, acting as a high-pass filter. The middle ear ossicles respond because of their moment of inertia, this response being modified by the tympanic membrane and the air enclosed in the middle ear, both acting like backsprings. The cochlear capsule undergoes distortional vibrations. The mass of the contained fluid, being unequally distributed, responds in an inertial manner. This cochlear response is modified by the oval and round windows which have not only different compliance values of their own, but face different impedances in the middle ear: the oval window, the ossicular chain; the round window, and the air enclosed in the middle ear.
Finally, along what might be called response line, i.e., the air in the external canal, the tympanic membrane, the ossicular chain, the oval window, and the cochlea, all of these various responses are collected and integrated with one another, according to their phase relationships, and the integrated response finally leads to hair cell stimulation, which creates the nerve impulses that go on to the brain via the auditory nerve.
Some researchers have previously demonstrated that a tone introduced simultaneously into the auditory canal and the skull (using a vibrator pressed against the head), with relative phase and magnitude adjusted properly, would result in no perception of sound in one or the other ear. However, there does not appear to have been any studies of the cancellation of more complex vibrational patterns inside the head. Some researchers have also demonstrated that some degree of reduction in structural vibration can be accomplished through active cancellation techniques. However, no such work on human structures has been found, and prior techniques require access to regions in which zonal nulls are desired.
Therefore, a need exists for a method and apparatus for nulling vibrational noise propagating through the bone structures and intracranial tissue in the head to the inner ear, and more particularly for a method and apparatus which will cancel vibrational noise in the inner ear zones transmitted to a patient from a medical instrument. The present invention satisfies that need, as well as others, and overcomes the deficiencies in prior methods and devices.